A slang word for a wonder bra. This is because it looks big and impressive from the outside and will cost you a fortune to get inside it. But once you actually get inside nothing else awaits but disappointment!

In a nutshell a 'wonderbra'.
Looking from the outside everything looks big and exciting, much like the millenium dome.
But once inside you're massively disappointed, and soon realise that it's nothing you're going to be writing home to your mates about.